Purpose of this project is to advance understanding of coagulation mechanisms that promote hemostasis in response to vascular injury but also cause diseases involving vascular thrombosis. In Aim 1, we will delineate a previously unrecognized convergence of extrinsic and intrinsic coagulation pathways differentially regulated by physiological and pharmacologic inhibitors and relevant to thrombin generation. This aim is predicated on the novel observation that, without thrombin feedback contribution, nascent FXa generated by TF-FVIIa can provide the activated cofactors, FVIIIa and FVa, required for assembly of functional intrinsic tenase (FIXa- FVIIIa) and prothrombinase (FXa-FVa) complexes, respectively. We will define the mechanisms underlying this newly recognized synergistic coagulation pathway studying reactions with purified components or in plasma; and we will establish how these mechanisms contribute to thrombus formation in blood flowing over TF and collagen as model thrombogenic surfaces, thus evaluating individual variability in response. In Aim 2, we will dissect the distinct contributions of different coagulation pathways to pro-hemostatic or thrombogenic processes. These studies stem from the observation that physiological coagulation inhibitors and FXa-directed anticoagulants differentially regulate the TF-dependent FXa activity generating FVIIIa and FVa as opposed to FXa required for prothrombinase function. Consequently, thrombin generation directly linked to TF pathway activation may be markedly inhibited while preserving TF-dependent FVIIIa and FVa cofactor production required for intrinsic tenase-driven prothrombinase activity. We hypothesize, therefore, that thrombin generation during hemostasis or thrombosis may involve a different balance of procoagulant and anticoagulant pathways in response to distinct vascular lesions and/or blood conditions that variably activate extrinsic and intrinsic coagulation pathways. Thus, we will evaluate the variability in the response of individual plasma samples to the anticoagulant effect of direct FXa inhibitors in clinical use - such as rivaroxaban and apixaban - and study mechanisms of thrombus formation elicited by cellular and matrix components of the vessel wall to define their relative contribution to coagulation initiation. Moreover, we will establish how different coagulation pathways, including the newly identified synergistic link between extrinsic and intrinsic coagulation, contribute to platelet thrombus formation and fibrin deposition induced by components of the atherosclerotic plaque. Recognizing the distinct coagulation pathways contributing to thrombus formation that may differentiate normal hemostasis from thrombosis will advance our understanding of serious pathological conditions resulting from vascular occlusion as well as excessive bleeding. Ultimately, our studies will have a positive impact on basic and translational research relevant to the prevention and treatment of socially relevant diseases associated with arterial and venous thrombosis or hemorrhage.